Wiped falling film evaporator



w. BUCHI ETAL,

WIFED FALLING FILM EVAPORATOR Filed Nov. 12, 1963 Dec. 20, 1966 Fig. 1

Heating Medium United States Patent O 3,292,683 WIPED FALLING FILM EVAPORATOR Walter Biiclli, In den Lohren, Flawil, St. Gallen, Switzerland, and Paul Janoslia, Ai Saleggi 4100, Bellinzona, Tessin, Switzerland Filed Nov. 12, 1963, Ser. No. 322,770 3 Claims. (Cl. 159-6) The present invention relates to a method for evaporating liquids in a thin layer evaporator, wherein the thin liquid layer to lbe evaporated is continuously moving downwards on a vertical heated surface, and means are provided to increase the kinetic energy of the thin liquid layer.

Increasing the kinetic energy of the thin liquid layer makes vortices in this liquid stream, so that heat transfer is improved and local superheating is lessened, i.e. vaporization of the liquid is promoted.

It is known how to produce a turbulent thin liquid layer by means of a scraper. A scraper deects the thin layer from its vertical direction of ow. A rapidly-moving scraper pushes the thin layer before it with its edge, so that a bow wave forms at the scraper edge. If a multblade rotating scraper is used, a bow wave forms before each -blade edge, so that there are several on the circumference of a cylindrical heating surface.

In such a case the pattern of the bow wave always coresponds to that of the blade edge. If the scraper blade adges are vertical, vertical bow waves are formed; if the blade edges are inclined to the vertical, bow waves inclined to the vertical are formed. The number of bow waves also lalways corresponds to the number of blade edges. If a helical blade edge is used, then a single helical bow wave will be formed along the total cylindrical heating surface.

This way of raising the kinetic energy of the thin liquid layer has many drawbacks. For constructional reasons the number of bow waves, that is to say the number of scraper blades, cannot be increased indefinitely. At high vrates of rotation the stress on the scraper due to hydraulic resistance is so great, that it must vhave a very massive construction. Consequently, the scraper has an important mass and must be well balanced dynamically in order to assure a steady running. Furthermore a large expenditure of energy is required to drive such a scraper. Because of the small number of bow waves there is only localised turbulence, i.e. the kinetic energy of only local places in the thin liquid layer is raised. The entire thin liquid layer cannot be stirred more strongly and equally at all places.

It follows therefore that using known methods, increase in kinetic energy with accompanying improvements in vaporization is possible only with large expenditure and to a very limited extent.

The principal object of the invention is the provision of a method without the said disadvantages. The method according to the invention in general consists in applying the liquid to be evaporated continuously the top of a vertical heated surface on which the liquid is moving downwards in the form of a thin liquid layer, imparting oscillating movements to several mechanical members separated from one another and extending over the entire breadth of the vertical surface to excite interference waves inside the layer, along and over the entire breadth of the vertical surface.

Another object of lthe invention is the provision of a a thin layer evaporator for carrying out the method according to the invention. In such an evaporator the thin liquid layer to be evaporated is moving downwards on the jacket innermost surface of a heated cylinder, and concentric to the cylinder and continguous to the thin liquid layer there is a helical band rotating about the axis of the cylinder. The helical band acts as a rubbing device for the thin liquid layer. On the one hand this helical band fixes the thickness of the thin liquid layer, and on the other hand it spreads the thin liquid layer equally over the surface of the cylinder. In addition the band acts as a dam wall for excess liquid running along the heated cylinder surface. Furthermore the helical band acts as a scraping orf device for residues on the heating wall.

On the other hand, according to the present invention, the helical band exhibits an axial oscillation to energize the thin liquid layer and thereby give rise to the formation of interference waves along and upon the entire circumference of the cylinder.

Hence the helical band in this invention forms an energy source for the thin liquid layer. In order to be able to use the helical band in accordance with the method of the invention, consideration must also be given to: the number of turns of the helical band, its internal elasticity, and the -kind of liquid generally to be evaporated. However, the invention shows the means by which the new method can `be applied.

The use, with a thin layer evaporator, of a rotor with bands, e.g. helicalns, placed over its extent, susceptible of oscillation and capable of being made to oscillate, has previously been proposed. However these serve to divide the liquid and to remove residues adhering to the heating wall. Several of these oscillating bands are arranged over the circumference of the cylinder. No interference waves could be formed in consequence, since mechanical members having axial oscillations are present .at only isolated points on the circumference. This energising of the thin layer at points cannot excite the entire cylindrical streaming thin liquid layer to interference oscillations. The known' oscillating bands will only hit the heating wall and remove the residues. By their turning the bands can yet push along a bow wave of the thin liquid layer.

One example of the invention will now be described with reference to the accompanying drawing, in which:

FIG. 1 shows a longitudinal section of a thin layer evaporator according to the invention; and

FIG. 2 is an enlarged section of FIG. 1, with interference waves.

The thin layer evaporator consists of a driving shaft 1, with a speed of rotation of about SOO-700 r.p.m. Shaft 1 is placed in bearing 2, and below has a bell 3. A helical band 4 is attached to this bell 3 by its upper end. The band `4 is rectangular in section. Also fastened to lche lbell '3 -is a hollow cylinder 5. This is somewhat longer than the helix 4, and over its entire surface it has a plurality of perforations y6; it can be said that cylinder 5 is perforated. The lower end of the helix 4 is fastened to cylinder 5. Bot-h cylinder 5 and helix 4 rotate with the shaft 1. A ring 4a is inserted at the top end and a ring 4b at the bottom end of the helix 4. The end windings of the helix are firmly connected to the rings 4a and 4b, respectively. The top ring 4a is xed on the cylinder 5, and the bottom ring 4b is made adjustable with respect to the cylinder S, either in angular position or in axial position. A set screw 4c or similar fastening means permits the fixing of the ring `4b in the desired position.

There are several vertical cooling tubes 7 and 8 inside cylinder 5. Tubes 7, 8 are fastened in a cap 9 above and in a plate 10 with an oblique surface, below. Let into the centre of the driving Shaft 1 is a bearing pin 11 fastened in the cap 9. The plate 10 is fastened in a housing part 12. Thus the cooling tubes 7, 8, along with ythe cap 9 and the plate 10, cannot be rotated. Three connections 13 to 15 end at the housing part 12. Further a tank 16 is fastened, by sealing, on the housing 12, and the cooling tube 8 and the connection 15 discharge into and out of the tank. The lower part of the thin layer evaporator has a housing hood 17, passing to an exit tube 18.

The helical band 4 can rotate inside a Vertical heating cylinder 19. Cylinder 19 is xed to a housing 20, 34. Housing 20,34 has four connections 21 to 24. A heating chamber 25 surrounding cylinder 19 ringwise, lies between the outer jacket surface of the cylinder 19 and the housing part 20. Connections 21 and 22 communicate with the heating chamber 25. The housing part 34 has inserted in it a ring 26, so that there is a ring gap 27 between ring 26 and the wall of the housing part 34. There is a liquid-space 28 between ring 26 and housing part 34. There is a Vapour space 29 between ring 26 and driving shaft 1, and `also between the housing part 34 and the driving shaft 1.

The mode of operation of the thin layer evaporator is as follows: Through connection 23 the liquid to be evaporated continuously ilows into the liquid space 28. From out of there the liquid runs through the ring gap 27 down wards onto the inner jacket surface of the heating cylinder 19, there forming a thin liquid layer 30 (FIG. 2). A heating medium flows through the connection 21 into the heating chamber 25, it heats the cylinder 19 and flows out again through connection 22. For better evaporation of this entire tubular thin liquid layer 30, it is set in oscillation (FIG. 2) bythe 'helical band 4.

The helical band 4 can -be regarded as several spreadout separated individual small pads of rectangular section, all the pads lying parallel to one lanother in the longitudinal direction of the helix 4. This picture of the individual small pads follows from the longitudinal section of helix -4 in FIGS. 1 and 2, if the section surface of helix 4 is regarded as consisting of individual pads. Now these pads are mechanical members, kextending along and over the entire breadth of the heating surface, that is over the whole inner jacket surface of heating cylinder 19. These mechanical members oscillate in a vertical direction, energising the thin liquid layer 30, so that interference waves 31 occur (FIG. 2). Graphically represented, ythe thin liquid laye-r 30 energised to interference oscillations thus appears as a cylinder of liquid whose jacket surface is shaped like corrugated iron, so that the waves surround the liquid cylinder ringwise.

The diameter of helix 4 is altered by twisting of the ends fastened to the bell 3 and the cylinder 5. At its largest ydiameter helix 4 lies on cylinder 19, at its smallest diameter on cylinder 5 (FIG. 2). The adjusting of the diameter of helix 4 and hence the adjusting of the thin liquid layer thickness depends on the nature of the liquid to be evaporated. By means not shown the ends of helix 4 can be brought closer to one another or separated. lengthening or shortening the helix changes the distance between the helical windings and can be adapted to the diiferent liquids to lbe evaporated.

The helical band 4 shown in FIG. 1 is made intrinsically of such an elasticity that at 600 r.p.m. it exhibits, with additional energising, an axial oscillation which excites the interference waves 31. FIG. 2 shows the thickness of the thin liquid layer ygreatly magnified.

A cooling agent flows into the thin liquid evaporator through the nozzle 13. The cooling agent flows upwards through tube 7 in the direction of the arrow, then down through tube I8, and flows out through connection 15.

When the liquid on its dowward movement along the cylinder 19, evaporates the lightest part flows into the vapour space 29 and hence out through connection 24. The next heaviest part passes through the perforations 6 in cylinder 5 and condenses on cooling tubes 7 and 8. This condensate runs along on tubes 7 and 8 and ows on the inclined plane of plate 10 out through connection 14. The heaviest part ows from the inner wall of heating cylinder 19 into the inside of housing bell 17, and thence as a concentrate in the direction of the arrow through the outlet tube 18.

In order that the lightest part can flow completely into the vapour space 29, the bell 3 has several bored holes 33. In order to achieve complete removal of the heaviest fraction from the heating wall, helix 4 is prolonged at the bottom out beyond the heating cylinder 19.

In the shown example, cylinder 5 serves to guide the helical band 4 and also as a screen between heating cylinder 19 and cooling tubes 7, 8. The vaporized fraction can ow radially inwards lthrough the perforations 6. Emission both from the heater cylinder and from the cooling tubes 7, 8 is however diminished by cylinder 5.

The illustrated thin layer evaporator is suitable yfor evaporating various liquids or liquid mixtures, since'the helix 4 can be altered somewhat in diameter and length. Cylinder S serves for this purpose. This cylinder 5 hangs xed only at its top to bell 3, and is free below. For evaporating only one liquid there can also be used a helix which cannot be changed in diameter and length. Then, the helix can still be fixed to bell 3 and hang freely below, while cylinder 5 is left olf. f

In the shown example, the axial oscillation of the helical band 4 due to its elasticity takes place at a speed of about 600 r.p.m., without the need of being excited to these oscillations by additional forces. If very viscous liquids are being evaporated, it is appropriate, as has been shown, to make the helical band stiffer and to induce an axial oscillation by additional forces. The helix can then oscillate in itself, but it can also be made to oscillate as a stiff body. To this end the helix is caused to oscillate vertically at suitable frequency and amplitude, electro-magnetically, hydraulically, or mechanically, whereby interference waves arise in the thin liquid layer.

The best rate of rotation exhibited by the helical band depends on the nature of the liquid used and on the time of throughput of the liquid through the evaporator. The speed'of the helix can thus reach about 2000 r.p.m.`

In FIG. 1 the helical band 4 can also be used hanging freely. Then the band is fastened only at its upper end to the bell 3, and hangs freely downwards over cylinder 5; it is not fastened below to cylinder 5. Then the endsof the helix 4 cannot be twisted, so that the diameter of the spiral cannot be varied. In this case the cylinder 5 acts only as a guide for helical band 4.

If the thin layer evaporator is built large, so that a helix 4 of large diameter and of relatively large mass is used, the surface of the heating cylinder 19 can be damaged by the scraping of the oscillating helix 4 on it. To prevent this, there can be used, for safety, a helix whose outer diameter can be diminished by pressure stress. If the helix is made, for example, of a band having round section (instead of the rectangular section of band 4 in FIGS. 1 ialud 2), the outer periphery of the helix can be ground at. a tubular jacket of elastically deformable material and round section, there is again a helix with windings of round section, but there is a space between the jacket and the yinner band. When the pressure of this helix against the heating cylinder 19 is high, -the outer periphery of the helical windings can elastically deform.

With the thin layer evaporator in FIG. l, the heating cylinder 19 can easily be interchanged. This is required because of wear on the cylinder wall, or when evaporating another kind of liquid using another kind of helix. The heating cylinder 19 can be pulled out of the evaporator axially and replaced by another by removing the housing part 34 upwardly and loosening the heating cylinder 19 from housing part 20.

When evaporating a liquid or liquid mixture by the method in accordance with the invention, the kinetic energy of the thin liquid lm can be much increased, without the need for -thick-walled and massive means as would be required with known apparatus. A very small radial clearance between heating and cooling surfaces is possible If the flattened band of the helix is surrounded by` due to the fact that the helical band 4 is very thin in radial direction of the helix, and this is the clearance between the thin liquid layer 30 and the wall of the cooling tubes 7. Consequently the vaporized part of the thin liquid layer 30 requires only a short distance to go until condensed. Because of -this short path very sharp separation is possible, as for instance the separation of highmolecular liquids into various constituents.

What we claim is:

1. An evaporator for evaporating liquids, comprising a vertical heated surface in the form of a cylinder of revolution, means for continuously applying liquid to the top of said heated surface to form a thin liquid layer moving downwards on said surface, a member contiguous to the thin liquid layer and extending over the entire height and breadth of the heated surface, said member being in the form of a helix lcoextensive with and having an axis which is coincident with the axis of said cylinder, said helix having windings which are capable to oscillate in the axial direction of the helix to excite interference waves inside said thin liquid layer along and over the entire height and breadth of the vertical surface, said vertical heated surface being constituted by the jacket innermost surface of a vertical cylinder, said helix being mounted for rotation about its axis, and means provided for rotating said helix, said helix being constituted by a helical lband anchored at and suspended from its top end.

2. An evaporator for evaporating liquids, comprising a Vertical heated surface in the form of a cylinder of revolution, means for continuously applying liquid to the top of said heated surface to form a thin liquid layer moving downwards on said surface, a member contiguous to the thin liquid layer and extending over the entire breadth of the heated surface, said member being in the form of a helix coextensive with and having an axis which is coincident with the axis of said cylinder, said helix having windings which are capable to oscillate in the axial direction of the helix to excite interference waves inside said thin liquid layer along and over the entire height and breadth of the vertical surface, said vertical heated surface being constituted by the jacket innermost surface of a vertical cylinder, said helix being mounted for rotation about its axis, and means provided for rotating said helix, said helix being in the form of a helical band fastened with its two ends exteriorly to a driving body which is axially aligned and augularly displaceable with respect to each other thereby rendering it possible to twist the two ends of the helix to change the diameter of the helix.

3. An evaporator for evaporating liquids, comprising a vertical heated surface in the form of a cylinder of revolution, means for continuously applying liquid to the top of said heated surface to form a thin liquid layer moving downwards on said surface, a member contiguous to the thin liquid layer and extending over the entire breadth of the heated surface, said member being in the form of a helix coextensive with and having an axis which is coincident with the axis of said cylinder, said helix having windings which are capable to oscillate in the axial direction of the helix to excite interference Waves inside said thin liquid layer along and over the entire height and breadth of the vertical surface, said vertical heated surface being constituted by the jacket surface of a vertical cylinder, said helix being mounted for rotation about its axis, and means provided for rotating said helix, said helix being in the form of a helical band fastened with its two ends to driving bodies which are axially aligned and axially displaceable with respect to each other to alter the length of the helix.

References Cited by the Examiner UNITED STATES PATENTS 1,403,804 1/ 1922 Merrell 159-6 2,542,270 2/ 1951 Zahm 159-6 2,656,885 10/1953 Hughes. Y 2,740,580 4/ 1956 Schmiedel. 3,004,901 10/1961 Merge et al. 159-6 X 3,054,729 9/ 1962 Smith 159-6 X FOREIGN PATENTS 1,348,930 12/ 1963 France. 1,136,982 9/1962 Germany. 1,154,438 9/ 1963 Germany.

330,805 6/1930 Great Britain.

656,605 8/1951 Great Britain. 38/14270 8/1963 Japan.

NORMAN YUDKOFF, Primary Examiner.

J. SOFER, Assistant Examiner. 

1. AN EVAPORATOR FOR EVAPORATING LIQUIDS, COMPRISING A VERTICAL HEATED SURFACE IN THE FORM OF A CYLINDER OF REVOLUTION, MEANS FOR CONTINUOUSLY APPLYING LIQUID TO THE TOP OF SAID HEATED SURFACE TO FORM A THIN LIQUID LAYER MOVING DOWNWARDS ON SAID SURFACE, A MEMBER CONTIGUOUS TO THE THIN LIQUID LAYER AND EXTENDING OVER THE ENTIRE HEIGHT AND BREATH OF THE HEATED SURFACE, SAID MEMBER BEING IN THE FORM OF A HELIX COEXTENSIVE WITH AND HAVING AN AXIS WHICH IS COINCIDENT WITH THE AXIS OF SAID CYLINDER, SAID HELIX HAVING WINDINGS WHICH ARE CAPABLE TO OSCILLATE IN THE AXIAL DIRECTION OF THE HELIX TO EXCITE INTERFERENCE WAVES INSIDE SAID THIN LIQUID LAYER 